1. Field of the Invention
This invention relates to a pressure water reactor (PR) with passive emergency cooling, and a method of operating the PWR in response to a loss of coolant accident (LOCA). More particularly, it relates to a PWR with a integral reactor pressure vessel in which the steam generators are contained within the pressure vessel, and which is provided with diverse arrangements for core and containment cooling and depressurization.
2. Background Information
PWRs have a reactor core of fissionable material housed in a reactor pressure vessel. In traditional commercial PWRs the reactor coolant, in the form of light water, is passed through the reactor core where it is heated by the fission reaction, and is then circulated in primary loops through steam generators. Cooled water from the steam generators is returned to the reactor pressure vessel and delivered to the reactor core inlet. The steam generators utilize the heat to generate steam that is delivered in secondary piping loops to a turbine generator to generate electricity.
The reactor pressure vessels and the steam generators of the PWR are housed in a containment structure that provides a barrier to the release of radioactivity. One of the concerns with a PWR is the possibility of a LOCA (loss of coolant accident) as the reactor coolant not only serves as a heat transfer medium, but also prevents the reactor core from overheating. A principal concern is a large LOCA that would result from the rupture of one of the large diameter pipes circulating reactor coolant through the steam generators. Such a leak would rapidly deplete coolant available to cover the reactor core as the pressurized coolant flashes to steam in addition to pouring through the rupture. Traditionally, the response to a large LOCA has been the injection of make up water to keep the core covered with water and cooled in order to remove heat that continues to be generated due to radioactive decay heat from the products of the fission reaction.
The trend is to provide passive systems, that is systems not requiring operator action or components requiring energization or components that once actuated require no additional energization or repositioning, to enhance reliability and safety. Several proposals have been made to integrate the steam generators into the reactor vessel. By placing the steam generators in the reactor pressure vessel, the principal cause of large size LOCAs is eliminated with the elimination of the large diameter piping between the reactor pressure vessel and the external steam generators.
There is still the potential for small or medium size LOCAs that must be addressed. One of the proposed integrated reactor designs, known as SIR (Safe Integral Reactor), utilizes a pressure suppression system as part of containment. In the event of a LOCA, coolant released as steam is quenched in water contained in a group of tanks. These tanks are cooled by natural circulation of ambient air. The containment is thus composed of the reactor cavity, a small dome structure over it, and the pressure suppression tanks and connection manifold. The quenching of the steam in the suppression tanks is designed to remove steam from the containment and thus to lower containment and reactor pressure. In order to replace water and water that flashes to steam and discharged from the reactor pressure vessel, steam injectors using steam from the integral reactor pressure vessel inject water from the suppression tanks into the reactor pressure vessel. The SIR circulates cooling water through an external heat exchanger and the secondary side of the steam generators during normal cool down after the turbine bypass system becomes ineffective, and this system appears to be sized only to absorb this normal decay heat at higher reactor pressures and temperatures.
A second integral reactor proposed by a Russian group houses the reactor pressure vessel containing the reactor core and steam generators in a small guard-vessel designed to withstand pressures from a LOCA up to 4 MPa. This system is directed toward maintaining sufficient core cooling by limiting the volume of the space available for escaping coolant and allowing the pressure to increase sufficiently to limit vaporization of coolant. The small guard vessel, and an extension housing other supporting equipment including condensers to limit the peak pressure that occurs and to provide long term pressure reduction capability, are both housed in a large containment structure. This integral reactor system also has heat removal loops connected to the steam generators providing passive heat removal in an emergency in addition to the above mentioned heat exchangers.
There is room for improvement in the structure and operation of PWRs with passive emergency cooling.
This invention is directed to a PWR, and a method of operating the PWR, employing an integral reactor vessel and having diverse emergency cooling systems that provide both core cooling and containment cooling following a LOCA. As one aspect of the invention, the PWR has a containment structure that allows the pressure in containment following a LOCA to rise while a heat exchanger connected to the secondary circuit of the steam generators in the reactor pressure vessel provides cooling of the reactor pressure vessel. The containment structure and the heat exchanger are sized to reduce the pressure in the reactor pressure vessel below the pressure in the containment structure to limit and actually reverse mass flow from the reactor pressure vessel within no more than about 3 hours following the LOCA without the need to add makeup water to the reactor pressure vessel in order to keep the reactor core covered with water and cooled. Thus, this arrangement results in the reduction in pressure within the containment structure without directly providing cooling of this structure and by condensing steam within the reactor pressure vessel replenishes the supply of water needed to cover and cool the reactor core. Preferably, the containment structure is spherical and is of moderate size to withstand the required pressure while reducing the space and cost of providing containment.
In order to minimize the size of the containment structure yet limit peak containment pressure, one or more suppression tanks are provided within the containment structure. Steam in the containment structure resulting from the LOCA is directed into and condensed by water in the suppression tank or tanks. Should the heat exchanger not be able to provide sufficient cooling to reduce the reactor pressure vessel such that the reactor core remains covered and cooled, the water in the suppression tank can be transferred, such as by gas pressure, to a flood-up cavity in which the lower portion of the reactor pressure vessel containing the reactor core is located to provide additional core cooling. The gas pressure is built up in the suppression tank by gas filling the containment structure which is forced into the suppression tank along with the steam by the initial high pressure in the containment structure resulting from the LOCA. Alternatively, or in addition, water in the suppression tank or tanks can be gravity fed into the reactor pressure vessel to keep the reactor core covered by mounting the tank or tanks above the reactor core. In an especially advantageous arrangement the gas from the containment structure and the steam resulting from the LOCA are introduced into the water in the suppression tank or tanks at a height which allows the water above the injection point to be transferred by gas pressure to a flood-up cavity in which the lower portion of the reactor pressure vessel containing the reactor core is located to provide additional core cooling when the pressure in containment falls, while leaving the remaining water for selective gravity feeding into the reactor pressure vessel. As a further alternative, water in the now flooded flood-up cavity will be at an elevation higher than the reactor core and can gravity drain into the reactor pressure vessel to keep the reactor core covered and cooled.
As an additional alternative cooling arrangement, a cooling fluid can be directed by a shroud over the external surface of the containment structure. This cooling of the external surface of the containment structure is limited if the heat exchangers reduce the reactor pressure vessel pressure and therefore containment structure pressure. However, in the event the heat exchangers do not remove heat and the containment structure pressure and temperature remain elevated, the external cooling is more effective and will limit the containment structure pressure to less than its design pressure thus providing a diverse means of limiting the containment structure pressure. As yet another alternative, the containment structure internal structure is arranged such that steam condensed on the inside surface of the shell cooled by external fluid will drain by gravity to the flood-up cavity. This condensed steam will flood the flood-up cavity to above the reactor core elevation such that the flood-up water can drain by gravity through a provided transfer path to the reactor vessel thereby keeping the reactor core covered with water and cooled. This method of cooling the reactor core together with the external cooling of the containment structure is diverse from the heat exchanger provided reactor core cooling.